1. Field of the Invention
The present invention relates to lithographic methods and, more particularly to lithographic methods for characterizing and monitoring lithographic exposure tool imaging performance.
2. Description of Related Art
Imaging performance, with respect to a lithographic exposure tool, is generally understood to describe an exposure tool""s ability to accurately produce an image of an object. The evaluation of this imaging performance is generally performed first to fully characterize performance over the tool""s exposure field and second to monitor tool performance, as part of a process control scheme, when the tool is used to produce images for the manufacture of, for example, an integrated circuit (IC).
For this first task, test pattern reticles or masks have been developed by lithographic tool manufacturers and users that have a number of specially designed test structures placed at a number of locations within the exposure field. An example of such a test pattern having nine groupings of test structures spread over an exposure field is seen, for example, in FIG. 14 of U.S. Pat. No. 4,908,656, issued Mar. 13, 1990 to Suwa et al., which is of different test structures formed therein are designed to enable the evaluation of the different factors that influence imaging performance.
Generally, the evaluation of imaging performance begins by exposing the test pattern at different locations on a substrate using a matrix of exposure conditions, for example as indicated in the exposure time vs. focus matrix depicted in FIG. 17 of Suwa et al. Once this matrix is completed, the various test structures are evaluated for each test pattern. While a variety of criteria are evaluated to fully characterize imaging performance, the measurement of the size of a test structure, referred to as its critical dimension (CD), is among the most important. As these test images are generally quite small, a CD of 0.25 micron (xcexcm) or less is typical for today""s high density ICs, a scanning electron microscope designed for such critical dimension measurements (CD-SEM) is generally used. In this manner, the tool""s imaging performance is characterized and a best set of exposure conditions selected.
Once the exposure conditions are selected and the lithographic tool begins to produce images in a manufacturing mode, imaging performance must still be evaluated to ensure that it remains within process control limits. Typically, users monitor imaging performance by measuring the CD of a test structure, developed for this second task, that is placed within the exposure field. In this manner, the test structure is present in each exposure field on the substrate or wafer and can be measured. As known, the number of measurements per wafer and the number of wafers measured per lot, or group of wafers, will vary as a function of the process control scheme employed. As mentioned for characterizing imaging performance, these in-process measurements are also generally performed using a CD-SEM.
However, as the size of the features of integrated circuits continues to shrink, the use of CD-SEMs for measuring critical dimensions becomes problematic for several reasons. First, as feature size becomes smaller, small changes in the size across an exposure field become more critical. For example, while a variation in size of xc2x10.01 micron (xcexcm) is only 1% of a 1 xcexcm feature, it is 10% of a 0.1 xcexcm feature. As a result, adequately evaluating imaging performance across an exposure field requires that more sites within each field be measured to allow for more critical adjustments to the tool. Thus the 9 sites shown in FIG. 14 of Suwa et al., are more often 15 or 18 sites now. Thus even with a state of the art CD-SEM, the 49 exposure fields in FIG. 17 of Suwa et al. at 18 sites per field adds considerable additional time to an evaluation. As this additional time can require additional measurement tools and as a state of the art CD-SEM is expensive, the purchase of any additional measurement tools can significantly impact manufacturing costs. A third problem is the ability of even state of the art CD-SEMs to resolve the test structures adequately to make accurate measurements as these structure""s sizes approach the resolution capability of the CD-SEM. A fourth problem, contamination caused by the electron beam, generally only affects the use of a CD-SEM for in-process measurements. Thus the very nature of bombarding a structure with an electron beam and the resultant secondary emissions that are captured for imaging, can also contaminate devices in the adjacent ICs. As a result, wafers used for in-process measurements as part of a process control scheme, are sometimes discarded or the photoresist layer removed, the wafer cleaned and reprocessed. Either alternative resulting in increased costs and reduced yields. Thus, alternate methods for characterization of imaging performance and in-process monitoring of that performance are needed.
One such alternate method reported to provide improved accuracy over CD-SEM measurements employs an electrical measurement of an array of test structures. The test structures of this method are formed in a conductive layer overlying a special test substrate where the structures have attached contact regions. Thus, a resist layer is exposed and the pattern of test structures developed and etched. After removing the resist layer, the conductivity of the etched features are measured by an electrical means. Using such parameters as the specific electroconductivity of the conductive layer and the length of the etched feature, a linewidth is calculated. This methodology has gained some acceptance characterizing a tool""s imaging performance, especially for small CDs. However, as the procedure involves a special substrate having a conductive layer and several processing steps after formation of an image to pattern the conductive layer, it is not suitable for in-process monitoring of imaging performance, and thus is limited in its use to tool characterization. As a result, an alternate method must be employed for in-process monitoring, thus requiring an additional step of determining the relationship between the electrical measurements and in-process monitoring measurements.
Thus it would be desirable to have a method of forming a test structure and method for using that test structure for both full characterization of imaging performance and in-process monitoring. In this manner the procedures that lithographic tool users employ are simplified. In addition, correlation between imaging performance characterization and in-process monitoring is ensured. It would also be desirable for the method of forming the test structures and method of its use provide for more accurate measurements of critical dimensions (CDs) then is currently possible. Additionally, it would be desirable if these methods are applicable for essentially any size CD. Finally, it would be desirable for this method to be cost effective, both in the cost of the processing required to form the test structures and the measuring equipment used to accurately measure those test structures.
In accordance with the present invention, a method for forming a critical dimension test mark (CDTM) and methods for both characterizing exposure tool imaging performance and in-process image performance monitoring, hereinafter referred to as process control monitoring, using the CDTM formed are provided. The embodiments of the present invention overcome the above and other drawbacks associated with prior art techniques for evaluating and monitoring the imaging performance of exposure tools.
The CDTM of some embodiments of the present invention is a doubly exposed region formed by superimposing a first feature or features on a reticle with a second feature or features on the reticle. In other embodiments of the present invention the first and second feature or features employed are generated from a data base used to drive the exposure tool, for example as in an electron beam direct write exposure system. Advantageously the CDTM of the present invention is formed using an exposure energy that is essentially equal to the energy needed to form a normal, singly exposed image.
The CDTM formed by the method of the present invention provides benefits that overcome the deficiencies of the prior art by essentially magnifying the size of prior art CD structures while maintaining an essentially normal exposure energy for the CDTM. In this manner the CDTM, formed by the methods of the present invention, are used to determine the critical dimension (CD) of a feature, for example a CD test structure in the manner of the prior art, without actually measuring that feature.
Thus in some embodiments of the present invention, first portions of an image forming layer overlying a conventional substrate are exposed with a first exposure energy. A conventional reticle is used having a first feature or features positioned with a first orientation to define the first portions. The first exposure energy is less than the nominal energy needed to fully form a normal, singly exposed image with the same reticle. In some embodiments, this first exposure energy is approximately one-half the nominal exposure energy. Second portions of the image forming layer are then exposed with a second exposure energy. This second exposure defines a second feature or features positioned at a second orientation such that an overlap region of the first and second features is formed. Advantageously, the sum of the first and second exposure energies is approximately equal to the nominal exposure energy. Hence, as only the overlap region is exposed during both the first and second exposures only this region receives sufficient energy to be fully defined in the image forming layer.
In some embodiments in accordance with the present invention, the first feature is repositioned to have the second orientation. In other embodiments, the first and second features are different test features of essentially the same size and shape. In still other embodiments, the first and second features are different test features of essentially the same shape but different size. Generally, the first and second orientations differ only by an angle of rotation about an axis that is within the doubly exposed or overlap region. Thus the overlap region, hereinafter referred to as the critical dimension test mark (CDTM), encompasses a region that has a dimension that magnifies the CD of the first and second feature. This magnification being defined by a relationship that comprehends the CDs of the first and second features, the angle of rotation between the first and second orientation and an empirical constant that includes the effects of the image forming layer and any processing required to define the CDTM within or on that layer.
It will be understood that while embodiments of the present invention have been described with reference to an image forming layer on a substrate, other methods of forming images can be employed and are within the scope of the present invention. For example, in some-embodiments of the present invention an image is formed using a image forming device rather than a layer, for example a charge coupled device array (CCD) or the like can be used to form an image of the CDTM and to characterize exposure tool imaging performance.
Embodiments of the present invention also include methods for using CDTMs formed to determine a critical dimension of a test feature without actually measuring that test feature. In this manner, not only can the imaging performance of an exposure tool over its exposure field be characterized, but also that imaging performance can be monitored while the exposure tool is used in a production mode. Advantageously, since the CDTM formed essentially magnify the CD of a test feature, for example by factors of approximately 10 or more, embodiments of the present invention provide characterization and monitoring data without the need to use an expensive CD-SEM. Rather, an optical apparatus or device, for example an apparatus that scans a laser beam or other optical beam over one or more CDTM""s and detects a signal caused by diffraction, reflection or scattering of the scanned beam from the CDTM""s can be used to measure the dimension of the CDTM. In some embodiments, the optical device for aligning a fiducial of a projected image from a reticle to a fiducial of a previous layer formed on the substrate is used to measure the dimension of the CDTM. As such optical devices are not typically employed as measurement tools, where such tools are used embodiments of the present invention include a software program to control the measurement process. In some embodiments of the present invention, the diffraction detecting device and control software employed for measuring the CDTM""s formed are incorporated within the exposure tool. In some embodiments, the optical measuring tool is a stand alone system.
It will be understood that while embodiments of the present invention have been described with reference to a device for detecting optical signals caused by diffraction from the CDTM of the present invention, other methods for measuring the size of the CDTM of the present invention are also appropriate. For example, in some embodiments of the present invention a measuring system employing a confocal microscope based detector is employed.